Function of the laryngeal mechanoreceptors during vocalization

The objective was to compare the activity of superior laryngeal nerve mechanoreceptors based on a respiration-based classification protocol with activities found using a vocalization-based classification protocol in adult cats.

Animal study.

In the first part of the experiment, single fiber action potentials from the internal branch of the superior laryngeal nerve were recorded in decerebrated cats using a respiration-based classification protocol to identify laryngeal mechanoreceptors as pressure, flow, and drive receptors, as described in the literature. A tracheal T-tube and a laryngeal mask airway were necessary modifications to perform this protocol. In the second part of the experiment, a vocalization-based classification protocol as described in the literature was used to classify the activity of the same fiber into the following groups: peak prephonatory, frequency-following, frequency-nonfollowing, inspiratory-modulated, or mixed. Vocalization was evoked by electrical stimulation of the midbrain in the region of the periaqueductal gray.

In all, eight cats were used for the experiment. Data were obtained from only three cats for a total of five single fibers. Results from five cats were not obtained because of inability to phonate (in three cats) and inability to record from the superior laryngeal nerve (in two cats). We identified two flow receptors, a drive receptor, a frequency-following receptor, and a frequency-nonfollowing receptor. Both flow receptor fibers were almost silent during the phonation phase and reached the maximum activity after vocalization during the inspiratory phase. The drive receptor was active during all four airway maneuvers and was most active during tracheal occlusion. It also kept a high level of activity during the phonatory phase, suggesting a role in the modulation of vocalization and respiration. The next two receptors, a frequency-following and a frequency-nonfollowing receptor, were active only during the phonatory phase and were totally inactive during the airway maneuvers, suggesting a role only during the vocalization behavior.

Because vocalization is an important stimulus for the activation of certain superior laryngeal nerve receptors, a classification protocol based on respiration alone is incomplete. Classification into pressure, flow, and drive receptors alone is not appropriate for the study of laryngeal receptors during vocalization. Some frequency-following and frequency-nonfollowing receptors may be active only during phonation and would otherwise be missed without vocalization stimuli.